Help design plywood box

I need to build a rectangular plywood box. The dimensions will be something like 12"x12"x48". One of the ends needs to be capped, but I could cap both ends. I was thinking about running 2x4's along the inside vertically and just nailing the plywood to them. I would then run small blocking perpendicular to the studs. This seems like a little overkill though. It will be used to support about 25 lbs.